It is proposed that out study on the histogenesis of the normal mouse brain utilizing the technique of autoradiography with tritiated thymidine as a cell marker be continued and that our beginning studies using the electron microscope to study the differentiation of neurons making up a selected neuronal population be continued, and that also with this technique efforts be directed to studying the migration of cells. It is also proposed that our studies of neurological mutants be continued. The purpose of these studies is to learn the principles governing neurogenesis and to provide developmental criteria for classifying neuronal systems. Answer to the following questions have been sought for the brain stem, colliculi, cerebellum, spinal cord, forebrain and sensory ganglia. 1. What is the time sequence of origin of neurons? 2. Where do the neurons of each cell population originate? 3. What is the pattern of their migration? 4. What is the differentiation pattern in neurons making up selected neuronal groups? 5. Is the time sequence of origin of neurons in neurological mutants different from the normal? If so, how? 6. Is it restricted to certain groups or wide spread? 7. What is the differentiation pattern in neurological mutants of neurons making up selected neuronal groups? How does it differ from the normal? Answers to question 1 have been completed for the brain stem, cerebellum and colliculi. The time of origin of the sensory ganglia is being determined. Answers to questions 2 and 3 have been completed for a number of nuclei of the brain stem, inferior colliculi and cerebellum. These studies are being actively continued. Answers to question 4 have been initiated and will be continued. The study of the neurological mutants has to date been restricted to study of the cytocytoarchitecture of the CNS of one neurological mutant.